Flexographic printing plates are well known in the art and are especially useful for commercial printing on diverse products such as flexible plastic containers, cartons, plastic bags, boxes and envelopes.
For the purpose of this specification, uncured plates to be used for preparing (cured) flexographic printing plates are referred to as flexographic printing plate precursors.
The flexographic printing plate precursors typically comprise a photo-curable layer prepared from a photo-curable polymer composition, and protective layer(s) to protect the photo-curable layer from daylight. Optionally, the flexographic printing plate precursor further comprises a support. A support is often used to give strength to the flexographic printing plate (precursor).
Alternatively, the side of the flexographic printing plate precursor opposite the image (printing) side of the printing plate is cured to form a support. Typically, in the absence of a support, a flexographic printing plate is prepared by first curing the side of the photo-curable layer of the flexographic printing plate precursor removed from the printing side. As a result that side of the photo-curable layer cross-links, becomes a thermoset substantially insoluble layer and acts as a support for the uncured part of the flexographic printing plate precursor. Subsequently, the side to be used for printing is selectively cured by exposing the photo-curable layer image-wise to light, e.g. UV light. The unexposed (uncured) parts of the layer are then removed in developer baths e.g. with a solvent or water. After drying, the flexographic printing plate is ready for use.
It will be appreciated that the image-wise curing of the flexographic printing plate precursor must be done in a precise manner. Any unintentional curing may lead to an unclear image on the flexographic printing plate, and, hence unclear prints.
In order to avoid undesired scattering of light it is important that the flexographic printing plate precursor is as transparent as possible, that is, not hazy.
Moreover, there is a need for further improvement of the total combination of physical properties of such photo-curable polymer composition for use in a flexographic printing plate (precursor).
Printing plates must be soft and elastic after curing. Normally, elasticity is obtained by the presence of an elastomer in the photo-curable polymer composition. Often, the elastomer is a block copolymer having two or more polymerised styrene blocks and one or more polymerised diene blocks.
The flexographic printing plate precursor must be easy and quick to cure, and the uncured material must be easy to remove. The flexographic printing plate must be dimensionally stable during storage; must be flexible enough to wrap around a printing cylinder; strong enough to withstand the rigors experienced during a printing process; abrasion resistant; soft enough to facilitate ink transfer during the printing process; and resistant enough to the particular ink solvent to avoid blurring of the image.
It will be appreciated that the achievement of an attractive balance of all these physical properties will be difficult. It would be desirable if a photo-curable polymer composition could be found that is less hazy than conventional photo-curable polymer compositions, and, preferably, has an attractive balance of other relevant properties, such as a low Shore A hardness.